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On two occasions, once a month ago and then today, my WRX has displayed a rough, high idle accompanied by surging throughout the rev range. Upon startup the idle is quite fast, 2-2.5k, and then "settles" to around 1.5k. If you rev the car or drive it for a bit, the idle "bounces" three or four times before settling in at the high 1.5k set point. The idle also sounds peculiar, like the car isn't firing on all cylinders (but don't take that literally since I don't know anything about mechanics).

When driving around the car surges: by this I mean that the car hesitates, stumbles a bit, then takes off. Occasionally the "taking off" part doesn't happen, and the car just stumbles to redline. The car really does not feel healthy, and it usually pulls like a beast.

The first time that this occurred (a month ago) the problem resolved on its own after driving the car on the highway for about 30 miles. This time it did not resolve through taking two 20 minute trips in the city with a brief jaunt on the highway. Also note that these two occurrences are separated by about 400 miles, so the gas clearly is from different sources each time. This last tank was from a Shell, I believe.

Relevant mods:

UTEC (VF34/FMIC/Ultimate 600cc injectors)
TurboXS BPV set to full recirculate
External wastegate plumbed into the exhaust
1 cat in the midpipe
Recent addition of TurboXS DTEC-BC Pro boost controller, but this first occurrence of the problem happened before this installation so I don't think it's related
I might have a small leak in the uppipe

What things should I ask the mechanic to focus on? From reading the old threads it seems these are the most likely culprits, no?

are you seeing a check engine light? make sure your bov is set to 100% recirc. i have an adjustable worx valve and at 50/50 it idled funny and lit a CEL.

no, i am not seeing a CEL. some more data: i have a knock light as part of the DTEC, and the car sometimes seems to have knock at times when it shouldn't: 3k, 4k rpm at light throttle, no boost, no load.

idle air controller sounds like the culprit..
take it off clean it and re-install.
but this means that you will have to replace
the sensor in the near future.

cool, thanks for the reply. not knowing exactly what the iacv does i wasn't sure if it could account for all of the symptoms. hopefully this will be an easy and cheap fix tomorrow (taking it to the dealer -- i'm too incompetent not to mention busy to work on the car myself, but not too busy to not able to post about it, go figure).

additional notes: the previous owner said he had to clean the iacv during the past year or so, and the car has ~75k on it, so i won't be heartbroken if i have to spring for a new iacv, even at $300.

I had the same problem with mine. I bought mine privately three months ago and its now sitting with a hole in the engine. I also have the Utec, fule rails, Boost controller etc .......... Long story short mine also shuttered while driving then gave me some power. It also stumbled to the redline and didn't take off properly. I pulled the exhaust manifold off and I was running extremely lean in two cyl. and rich in the other two. Also, I noticed the boost levels maxing out higher towards the end of the third month and oil drops coming out the exhaust. It's apparent the guy before me who modified it beat the piss out of it and now needless to say it has a 2" x 2" hole in the back of the engine after I have had it for only 3 months and I have not abused it since I have owned it.

I work for Subaru and the techs who work at my dealership looked at it and said it was definitely beat. Guess I shouldn't have been so trusting of my friend's advice since he didn't know the guy who owned it before him when he said it wasn't abused. Now I have a Subaru that needs over $3500.00 worth of engine block, gaskets, fluids, tuning and work!!!!

well, i took it to the dealership today and they said that they couldn't work on it since it is so modified. however, they did pull the codes at least (i should have just gone to autozone...): temp sensor, cat insufficiency (makes sense as i'm only running one), and misfires on all four cylinders. it was also noted that the IACV's water line (?) wasn't functional, for what that's worth.

on the other hand, while driving to the dealership the idle problem went away, just as it did the first time this occurred a month ago. it might have gone away due to chance, due to bad gas being worked out of the system, due to the bottle of injector cleaner i dumped in yesterday, or because it knew it was on the way to the dealership... who knows.

in any case the car is pulling strongly once again so i'm going to keep my appointment with pdxtuning this weekend and hope for the best.

It always had the misfire codes. Some people say underdrive pulleys or lightened flywheels can cause misfire CELs, and you car has both. Sounds like the IACV, but it's odd it didn't throw a code. I had it do the same thing on me...I took the IACV out and cleaned it, car ran fine after that. You WILL need a new gasket for it though, so don't take it out until you have a new gasket. Also, the coolant line you're talking about it for the throttle body. I have the throttle body bypassed in the cooling system since all it did here was heat up the throttle body. You may need to reconnect it if you have problems on really cold days.

you have an A4 now, eh? it's throwing an idle air insuffiency code now, too, so i think the iacv may be going out/need cleaning again. in any case the idle problem is gone somehow, might also have been the fuel filter that i replaced today.

the car has been cured of all its part throttle weirdness now, by the way, by a tune from the fine folks at PDX Tuning. the tuner wasn't too impressed with your fuel map, Chris... 283 whp and 262 ft-lbs were the final numbers on a dynojet, and that's with the wastegate plumbed to the downpipe, a cat in the midpipe, a resonator, and a prodrive axleback (but also with a DTEC-BC Pro with two solenoids and a non-slipping clutch).

maybe it was due to the differences in exhaust setup but it wasn't working great for me, especially chugging around town and in the mid-rpm mid-tps part of things. air:fuel was just all over the place even if it did pull well at full throttle.

i ended up with an ACT street disc, and the pdx folks haven't sent me the graph yet. i'll post it once i get it.